1. Technical Field
This invention relates to the fabrication of lithographic masks and, in particular, the fabrication of lithographic masks utilized in device fabrication.
2. Art Background
In the fabrication of devices, e.g., semiconductor devices, magnetic bubble devices or optical devices, it is generally necessary to configure on a substrate a region (e.g., a metal, semiconductor or dielectric region) in a specific spatial pattern and location. (A substrate is a mechanically stable body including, e.g., semiconductor regions and/or metal regions and/or dielectric regions.) The positioning and/or patterning of these regions is generally accomplished by a lithographic process. In this process a mask is utilized to image energy in the desired pattern onto a substrate surface that has been coated with a material sensitive to the incident energy. The mask in this exposure step is generally placed in contact with or in close spatial relation to the substrate. Alternatively, the mask pattern is projected onto the substrate.
After exposure, development of the energy sensitive material is performed to selectively remove either the exposed, or unexposed regions, of the resist material. (For a negative resist the unexposed region is removed while for a positive resist the exposed region is removed.) Generally, a solvent or energetic entities from a plasma are employed to effect this removal. The resulting, patterned energy sensitive material, i.e., resist, is employable as a processing mask for accomplishing the processing, e.g., selective doping, etching, oxidizing of or deposition onto the underlying substrate regions.
The mask utilized for exposure of the resist material generally includes a patterned metal or metal oxide film. Materials such as chromium, chromium oxide, tungsten or nickel are typically used for photomasks. These materials are commonly formed in a layer thickness of approximately 500 Angstroms to 1000 Angstroms for photomasks on a transparent substrate such as a quartz glass substrate. (In the context of this disclosure, the terms transparent and opaque refer to the energy that is effective in inducing reaction in the resist material to be exposed. For a material to be considered opaque, it should transmit less than 10 percent of this energy while for the material to be considered transparent it should transmit at least 80 percent of this energy.) The metal or metal oxide film of the mask is typically manufactured by depositing a resist material sensitive to electrons onto its surface, exposing this resist material with a directed electron beam, and developing the exposed resist to form the desired pattern (see, D. J. Elliott, Integrated Circuit Fabrication Technology, McGraw-Hill, New York, 1982, for a description of the fabrication of photomasks).
In the manufacture of masks, transparent defects such as pin holes or entire missing portions often occur. These defects, in turn, cause defects in the integrated circuit or other device produced when using the mask. Since the manufacture of masks is generally a time-consuming and relatively expensive operation, it is often desirable to repair a defective mask by selectively forming an opaque material on the unwanted transparent region. The repair procedure is, however, not acceptable unless it is less costly than merely producing another mask. The repair should also produce an opaque deposit that is sufficiently adherent to the mask substrate that subsequent processing and cleaning during mask fabrication or during subsequent use of the mask does not induce loss of the repaired material. Additionally, the resolution of the repair procedure should be at least as good as the desired resolution of the mask itself to avoid mask and, in turn, device degradation.
A variety of processes have been disclosed for effecting repair of transparent defects. In one procedure, such as disclosed in U.S. Pat. application Ser. No. 735,851 dated May 20, 1985, which is the parent of U.S. Pat. application Ser. No. 086,210 dated Aug. 17, 1987, a laser is utilized to expose a metal ink on the mask surface. Although this procedure has been found quite efficacious for repair of masks formed with design rules of 1 .mu.m or larger, repair of masks with finer design rules, e.g., 1 .mu.m or finer, is more difficult. (A design rule is the feature size for the smallest feature (line or space) that must be delineated by the mask to form the desired device.) Another procedure, formation of carbonaceous films utilizing a focused ion beam, has also been discussed in H. C. Kaufman, W. B. Thompson, and G. J. Dunn, Proceedings of SPIE, International Society of Optical Engineering, 632, 60 (1986) and M. Yamamoto, M. Sato, H. Kyogoku, K. Aita, Y. Nakugawa, A. Yasaka, R. Takusawa, and O. Hattori, Proceedings of SPIE, International Society of Optical Engineering, 632, 97 (1986). However, essentially no details have been given concerning the conditions or materials utilized and these processes have been difficult to reproducibly control.